docs: def find_lexer_class(name):
      """Lookup a lexer class by name.
  
      Return None if not found.
      """
      if name in _lexer_cache:
          return _lexer_cache[name]
      # lookup builtin lexers
      for module_name, lname, aliases, _, _ in itervalues(LEXERS):
          if name == lname:
              _load_lexers(module_name)
              return _lexer_cache[name]
      # continue with lexers from setuptools entrypoints
      for cls in find_plugin_lexers():
          if cls.name == name:
              return cls